Payment does not reflect on balance

I agree that you pay a certain amount of interest, but obviously not the full payment amount is interest. My petrol card payment does reflect on the statement, but the balance remain the same as the pervious month. Meaning that either Nedbank's system is faulty and cannot calculate properly, or Nedbank is overcharging on interest. I also see a lot of complaints about unauthorised debit orders, I also experience problems with unauthorised debit orders. Paid R45.00 to stop the debit order and still needs to phone Nedbank monthly to reverse the debit order. Nedbank's response is that they cannot stop it if the description or amount change. Now the question is, how safe is our money if Nedbank allow this to happen and cannot stop it, or is this a nice way to overcharge on bank cost? Please fix the problem as a matter of urgency?